Communications systems require radio frequency amplification for increasing system sensitivity when receiving low level signals from an antenna located at the top of a radio tower.
A tower mounted preamplifier located at the base of an antenna may offer up to a ten DB improvement in receiver sensitivity over preamplifier systems located at the base of the tower in that the former preserves the signal-to-noise ratio existing at the base of the antenna by amplifying signals before they are reduced in amplitude by tower-to-ground transmission line losses. Unfortunately tower mounted preamplifiers are not readily accessible for maintenance in case of system failure. Therefore it has been a practice to provide systems which permit the tower mounted preamplifiers to be bypassed and signals carried by the transmission line to a backup or secondary preamplifier located at ground level. When the system is switched to the backup preamplifier, which is not located on top of the tower, the improvement in system sensitivity achieved by using a tower mounted preamplifier is lost.
Radio frequency amplifiers may be used in two-way repeater amplifiers, i.e. bidirectional amplifiers, or one-way amplifiers. In critical system applications where amplifier failure caused down time is a major concern, a backup circuit is usually incorporated in the system.
The use of an alternate amplifier circuit to be used in the event of a primary circuit failure is well known but all of the existing systems fail to provide a backup system suitable for a tower mounted application with remote control. For instance, J. Maggio et al, U.S. Pat. No. 2,229,108 on "Switching Of Spare Repeater Sections" issued Jan. 21, 1941 discloses the broad concept of a spare amplifier circuit in combination with a plurality of amplifier circuits. The system requires an intricate array of relays to provide a single spare circuit for a three circuit system and the extensive hardware required to provide the switching increases the probability of failure to a point which far exceeds any benefits which may be derived in overall reliability from inclusion of the spare circuit. An even more serious shortcoming of the Maggio et al system is the fact that the multi-pole, multi-throw relays incorporated in the system are satisfactory for high frequency operation but completely unsatisfactory for use with preamplifiers where RF frequencies are encountered.
Other examples of backup amplifier circuits may be found in Haddock, U.S. Pat. No. 1,396,745; Blattner, U.S. Pat. No. 1,472,455; Toomy, U.S. Pat. No. 1,525,054; Daly, U.S. Pat. No. 2,647,176; and Karlson, U.S. Pat. No. 2,773,944. None of the techniques utilized by these references lends itself to the use in a remote tower or to preamplifier systems operating in the RF range.
In the forgoing systems, the "standby" method has an advantage over the "common redundant preamplifier" method. The redundant method connects two preamplifiers in parallel with input and output 3 db hybrids. The theory behind this is that if one amplifier fails, the second amplifier will maintain system operation, although at a reduced performance level due to an increase in noise and reduction in gain. Another drawback of the redundant method is that both preamplifiers are active in the system and both are vulnerable to damage by lightning.
Many of the above problems are eliminated by the teachings of U.S. Pat. No. 4,565,972 for a "Tower Mounted Preamplifier", a preamplifier backup system which is located in the tower but activated at ground level by interrupting the DC voltage to the tower mounted preamplifier assembly for a short time duration. This operates a latching relay which switches DC power and RF signals alternately between either of two identical preamplifiers each time the power is interrupted. This is usually done manually at the site after poor signal reception has been experienced, although the switching could be done remotely via a modem and telephone line or other radio link to the modem.
Following the teachings of U.S. Pat. No. 4,565,972 requires an action to be taken by an individual to place the standby or secondary amplifier in operation.